1. Field of the Invention
The present invention relates to a polarizing plate and a manufacturing method thereof. The polarizing plate may be used alone or as a part of a laminated optical film to form liquid crystal displays (LCDs), organic EL displays, CRTs, PDPs, and so on.
2. Description of the Related Art
The liquid crystal display market has rapidly expanded in such fields as clocks, mobile phones, PDAs, note PCs, PC monitors, DVD players, and TVs. Liquid crystal displays use liquid crystal switching to visualize changes in polarization state, and based on the display principle, they use polarizers. Particularly in TV applications and the like, there is an increasing demand for higher brightness, higher contrast, and wider viewing angle. Thus, higher transmittance, higher degree of polarization and higher color reproducibility are also demanded of polarizing plates.
Iodine containing polarizer made of stretched polyvinyl alcohol in which iodine is adsorbed has high transmittance and high degree of polarization and are most popular polarizers widely used. A general polarizing plate is manufactured by bonding a transparent protective film to both sides of a polarizer with a so-called aqueous adhesive, which is a solution of a polyvinyl alcohol material in water (see JP-A Nos. 2006-220732 and 2001-296427). Triacetylcellulose or the like having high moisture permeability is used for the transparent protective film.
When an aqueous adhesive such as a polyvinyl alcohol adhesive is used in the process of manufacturing the polarizing plate as described above, however, a drying process is necessary after the polarizer and the transparent protective film are laminated. The presence of such a drying process in manufacture of the polarizing plate is unfavorable in view of an improvement in the productivity of the polarizing plate.
Also when an aqueous adhesive is used (in the case of so-called wet lamination), the polarizer must have relatively high moisture content such that the adhesion to the adhesive can be increased (in general, the polarizer has a moisture content of about 30%). Otherwise, the aqueous adhesive cannot provide good adhesion in the resulting polarizing plate. However, the polarizing plate obtained as described above has a problem in which its dimensions can significantly change at high temperature or at high temperature and high humidity. On the other hand, in order to suppress such a dimensional change, the moisture content of the polarizer may be reduced, or a transparent protective film with low moisture permeability may be used. However, the lamination of such a polarizer and such a transparent protective film with an aqueous adhesive can lead to a reduction in the efficiency of the drying process, a degradation in the polarizing properties, or defects of appearance so that practically useful polarizing plates cannot be obtained.
In recent years, as the screen size of image displays (particularly typified by TVs) increases, upsizing of polarizing plates becomes very important in view of productivity and cost (an increase in yield and the number of available pieces). However, the polarizing plate using the aqueous adhesive has a problem in which a change in its size occurs by the heat from a backlight to cause unevenness so that the so-called light leakage (unevenness) can be significant in which a black viewing displayed on the whole of the screen partially looks whitish.
Under the above-described circumstances, there are many proposals for the use of active energy ray-curable (particularly ultraviolet-ray curing) adhesives in place of aqueous adhesives. For example, an ultraviolet-ray curable adhesive is proposed that includes an acrylic oligomer of epoxy acrylate, urethane acrylate, polyester acrylate, or the like, and an acrylic or methacrylic monomer as a diluent (see JP-A No. 61-246719). In a polarizing plate including a polyvinyl alcohol polarizer and a transparent protective film bonded together with an active energy ray-curable adhesive, however, a significant reduction in transmittance and red discoloration are observed under heating conditions, and therefore, it does not have satisfactory durability at high temperature.